In radiation therapy, spatially targeted dosages of ionizing radiation are applied to a tumor or other region containing cancerous or malignant tissue. Growing and rapidly multiplying cancer cells are often more susceptible to damage from ionizing radiation as compared with normal cells, and so enforced by the higher dosage administered by proper planning, the applied radiation preferentially kills cancerous or malignant tissue. Nonetheless, ionizing radiation is harmful to both malignant and healthy cells, and so precise spatial targeting of the radiation is important for limiting collateral damage to healthy tissue. Damage to healthy tissue is also typically controlled by performing the radiation therapy in a series of sessions extending over several days, weeks or months, with the expectation that radiation damage to healthy tissue may undergo healing during the time intervals between treatment sessions.
In radiation therapy, the radiation beam is applied at angular positions around the subject in a manner that combines to produce a targeted total radiation dosage spatial distribution that is concentrated on the tumor or other region to be treated. Angular coverage can be achieved by using several stationary radiation sources distributed around the subject, or by revolving a radiation source such as a linear accelerator (linac) around the subject (i.e., tomotherapy). The radiation therapy is planned in advance for a specific subject, based on imaging data for that subject acquired using computed tomography (CT), magnetic resonance (MR) imaging, or another suitable imaging modality. To achieve sufficiently accurate radiation targeting, the planning is performed using images acquired for the specific subject to undergo therapy. The tumor or other target is delineated in the images, along with delineation of critical organs or regions where radiation dosage must be limited. Computer simulations or other electronic calculations are used to optimize the spatial profiles and intensities of the radiation beam to concentrate the radiation in the target while minimizing exposure of critical but possibly normal organs or regions.
In most existing radiation therapy facilities, the CT, MR, or other imaging system used for the planning is physically separate from the radiation therapy system that includes the ionizing radiation source or sources. The spatial frame of reference of the imaging must then be registered with the spatial frame of reference of the radiation therapy system, for example using indicator markers glued to the subject. Registration errors result in less well targeted radiation therapy, which can reduce its effectiveness at combating the cancer and can increase collateral damage to healthy tissue. Another problem is that the time between planning data acquisition and radiation treatment can be many days or even longer. In the interim, patient anatomy may shift or change due to bladder filling, bowel motion, weight loss or gain, or so forth.
Still further, although the subject is generally immobilized during the radiation therapy treatment, various kinds of subject motion can still occur during the radiation therapy, such as respiration, bulk subject motion, or so forth.
In view of these issues, it has been recognized that it would be advantageous to image the subject using an imaging modality physically connected with the radiation therapy system. Various such “hybrid” systems have been proposed, which combine a CT or MR system with a radiation treatment system. In some MR imaging-based arrangements, the MR scanner magnet is divided into two halves defining an axial gap that receives the radiation therapy system or that allows the radiation therapy beam to pass (see, e.g. Demsey, U.S. Publ. Appl. No. 2005/0197564 A1; Green, U.S. Publ. Appl. No. 2001/0001807 A1; Fallone et al., WO 2007/045076 A1; Lagendijk et al, WO 2004/024235 A1). Other MR scanner components, such as gradient coils and radio frequency coils, can be similarly divided or arranged to lie outside of the gap, or can be made sufficiently thin or of suitable materials such that the therapy beam can pass through the components without substantial attenuation. In such a system, both the imaging and the radiation therapy modalities share the same system of coordinates, which removes or simplifies the registration component of the radiation therapy planning process. Additionally, radiation therapy planning can be performed immediately before each radiation therapy session, thus minimizing error due to changes in patient anatomy between the planning and therapy.
To address possible subject movement during the radiation therapy session, it has been contemplated to use such a hybrid system to perform imaging during the radiation therapy session, so as to detect subject motion. However, difficulties arise in performing concurrent imaging and radiation therapy. Computed tomography (CT) scanners are not well suited for such concurrent operation, because it is difficult or impossible to physically arrange for the x-ray beam of the CT scanner to simultaneously probe the spatial region undergoing radiation therapy. MR has been recognized as a better imaging modality candidate for concurrent imaging. However, interference caused by the radiation therapy system has heretofore been found to prevent acquisition of quality MR images during the radiation therapy.
Fallone et al., WO2007045076 has proposed to interleave MR imaging and radiation therapy operations by preventing linear accelerator (linac) pulsing during the MR acquisition window. This means that radiation therapy is not performed during the imaging. Even for rapid real-time three-dimensional imaging sequences, the number of imaging data sampling intervals is sufficiently large that the duty cycle of the radiation source under this approach is substantially extended, thus prolonging the therapy session.
The following provides new and improved apparatuses and methods which overcome the above-referenced problems and others.